Final answer:
Fat yields more energy per gram than carbohydrates and is the preferred fuel during low-intensity aerobic activities due to vast reserves, while carbohydrates are used for higher intensity activities as they are more oxygen-efficient but can deplete quickly.
Step-by-step explanation:
The difference between fat and carbohydrates (CHO) in the aerobic system revolves around their energy yields and efficiency in oxygen use. Fat is a long-term fuel source that produces more ATP, due to its highly reduced form, yielding more energy per gram (option c). It is used primarily during low to moderate intensity exercises. Conversely, CHO is the primary source of energy for high-intensity activities and is more oxygen-efficient. The glycogen form of carbohydrates is hydrophilic and its stores in the body can be depleted, leading to fatigue. In contrast, fat reserves are vast, enabling longer-term energy supply during aerobic activities (as long as oxygen is present).